This invention concerns a method and system for recycling volatile organic compounds (VOCs) used in solvent-based paints. The system and method is applicable whenever a closed chamber or booth is used for spray-painting a workpiece, such as a car, to eliminate venting of VOCs to the atmosphere while capturing the VOCs for recycling.
Present day paint spray-booths need to be designed to provide a safe working environment, to prevent pollution of the atmosphere, and to enhance the quality of the product being sprayed. In general, paint spray-booths are equipped with a constant flow of air into and out of the spray-booth to maintain a reduced level of VOCs in the internal atmosphere. This arrangement has led to a number of attempts to reduce the level of VOCs in the exhaust air to minimize environmental pollution.
One such attempt involves the use of aqueous-based, rather than solvent-based paints. Such efforts have generally produced inferior paint results and excessively long periods for drying of the workpiece. Limiting the amount of VOCs in solvent based paints, as reported by at least one manufacturer, does help to reduce VOC emissions. However, substantial VOC emissions still occur when using such paints, and a more comprehensive solution is required.
Another attempt is found in U.S. Pat. No. 4,261,707, which discloses a complex process and system for elimination of paint solvent vapors from an automobile paint spray-booth and curing oven to maintain a healthful and clean environment for workers in the paint booth. The process and apparatus involves vacuum assisted vaporization of solvents from scrubber water for subsequent condensation and recovery of the solvent.
In U.S. Pat. No. 4,444,573, a method was disclosed for using a hydrotropic substance to decrease the level of an organic solvent in an air stream from a paint spray-booth. A hydrotrope is defined as a chemical substance which includes an organic group chemically bonded to a polar group. The function of the hydrotrope is to increase the solubility of a volatile solvent in water. The method involves (a) contacting of solvent laden air from a paint spray-booth with a spray of aqueous hydrotrope, (b) extracting solvent from the aqueous hydrotrope using an organic oil, and (c) stripping the solvent from the oil using a steam stripper followed by condensation and recovery of the solvent.
In U.S. Pat. No. 4,620,858, related to the U.S. Pat. No. 4,261,707 patent discussed above, a method and system is disclosed which achieves contact between solvent from paint spray and an organic solvent absorbing liquid, followed by regeneration of the solvent absorbing liquid. This contact is achieved in a chamber separate from the chamber in which the spraying operation is conducted.
In U.S. Pat. No. 4,927,437, a method and apparatus for removing particles, including solvent droplets, from moving air in a paint spray-booth. The apparatus involves a cyclonic separator.
In the instant invention, a closed system is provided such that, in general, air is not vented to the atmosphere during paint spray operation. Rather, a worker in the closed system is provided with an independent source of air, such as grade D fresh air supplied respirators, thus complying with health and safety requirements of the Occupational Safety and Health Administration (OSHA). The use of a supplied air respirator is already mandated by OSHA (29 CFR 1910.134d1). The instant system involves direct condensation of VOCs from the humidified paint booth atmosphere and direct condensation of solvent laden water vapor, rather than vacuum assisted vaporization and subsequent condensation. A controlled temperature and humidity environment for optimal painting conditions is thereby provided while at the same time reducing environmental pollution.